Aromatic hydrocarbons such as BTXs have heretofore been put to the refining treatment in order to remove unsaturated hydrocarbon compounds such as olefins and diolefins therefrom. The refining treatment is to remove, from the aromatic hydrocarbons, the unsaturated compounds such as BTXs as high-boiling fractions of distillate by polymerizing them into dimers or trimers thereof or by alkylating them into aromatic hydrocarbons so as to transform them into polycyclic aromatic compounds of high molecular weights. It has been known that during these reactions, there takes place disproportionation reaction or isomerization reaction of the alkylaromatic compounds such as toluene and xylene.
The activated clay has heretofore been used for the refining treatment from such a standpoint that it works as catalyst for polymerizing unsaturated hydrocarbons (see, for example, patent documents 1 and 2). Though the activated clay is used for decolorizing mineral oils, it has been known that the activated clay for decolorization can also be used for the refining of aromatic hydrocarbons (patent documents 3 and 4).